The present invention relates to a device for transmitting light to and receiving light from a remote sample for analysis and more particularly to an optical probe for use in measuring the optical response of a remote sample.
The ability to monitor particulate matter in process streams and emissions to the air from industrial operations, and particularly the ability to do so in-situ and in real-time, is becoming increasingly important in many industrial processes. This is the case not only because of the desire to control and modify various processes in real-time to improve their efficiency but also to comply with various environmental regulations governing the composition, quantity and quality of industrial emissions.
Air emissions of toxic, hazardous and regulated materials are coming under increasing scrutiny by the regulatory community. Not only are industrial operations being required to monitor their air emissions more closely but also they are being required to do so on a continuous basis. Measurement of hazardous metal concentrations in stack emissions is a difficult task. Currently, air emissions of these metals from industrial operations are measured using extractive sampling followed by off-line chemical analysis, a procedure that is costly and typically has long turnaround times. Furthermore, because of the many manual operations involved in extractive sampling there is a significant potential for introducing sampling errors. Complete analyses of stack measurements typically are not available for two to four weeks from the time that samples are collected. Furthermore, certification tests require that more than one sample be taken for a given operating condition and that at least one sample be taken for each operating condition. The long turnaround times inherent in extractive sampling prevent the use of air emissions measurements as a method of controlling operating parameters in real-time. Continuous measurements of industrial air emissions could ultimately provide real-time information that could be used by facility operators to modify operating parameters to improve efficiency or reduce air emissions. Although most of the metal air emissions are in the particulate phase, vapors may also be significant and must be measured simultaneously. Furthermore, the particles that contain metals may be quite inhomogeneous and particulate metals may be present in any of a large number of compounds.
Optical methods, because they can provide instantaneous data readouts, can generally be located within or in proximity to a flow stream (process stream or source of air emissions) and can be placed in remote locations, are particularly useful as a means of monitoring particulate air emissions and controlling operating parameters. Optical methods can be classified into two principle classes: one, wherein an output optical signal, produced in response to an input optical signal, is measured; two, wherein a change in the input optical signal, produced in response to the medium through which the input optical signal has traveled, is measured (i.e., a transmittance measurement).
A wide variety of instruments are currently available for on-line analysis of flow streams. However, the optical probes that these instruments use are typically designed for analysis of the concentration of constituents in fluid streams. These optical probes generally contain bundles of optical fibers and specialized lenses and mirrors that are not suitable for use in the harsh environments encountered in monitoring particulate emissions from industrial boilers, incinerators and furnaces. Furthermore, many of these instruments employ beam dividers or splitters, an arrangement which causes more than 75% of the available light to be lost. Because of the requirement for a second probe that receives light transmitted through the sample, instruments that operate in the transmittance mode are generally unsuited for use in the harsh environments of stack emissions from boilers, incinerators, furnaces and the like.
A method for circumventing many of the problems discussed above was disclosed in U.S. Pat. No. 4,637,716. Here, an anemometer measures light scattered from particles in a fluid, wherein an entrant light beam from a laser passes through a hole in a mirror inclined 45.degree. to the beam axis and is focused by a focusing lens onto the end of an optical fiber. Scattered light collected by the optical fiber emerges from the fiber at a larger angle than the entrant light and is converted to a relatively large diameter beam by the focusing lens. The parallel beam of returning light is subsequently reflected by the inclined mirror into a detection system via a second lens system. While the method of getting light into and out of an analysis area described in the '716 patent overcomes many of the deficiencies noted earlier, this method does not permit focusing the entrant light beam on any particular area or particle selected for analysis within a flow stream. Furthermore, while optical fibers are useful for transmitting an incident light beam from a low power (.about.1-2 Watt) laser they are completely unsatisfactory for transmitting the high intensity incident light beam from a high power (.about.kilowatts) laser such as would be used for laser spark spectroscopy, for example, because of severe degradation of the optical fiber by the high intensity laser light.
For the reasons set forth above, it is highly desirable to have an optical probe that permits measurements to be made at a plurality of locations within a flow stream, is rugged enough to be used for monitoring emissions from industrial boilers, incinerators and furnaces and can introduce the optical input to the measurement location and extract the optical response through limited access ports in a chamber or duct enclosing the emissions. It is further desired that the probe should be reliable, suitable for remote sampling and easy to align and operate.
The instant invention provides an optical probe whereby all of its optical components (source, detector, relay optics, etc.) can be located in proximity to one another and generally exterior to the flow stream being monitored thereby permitting a compact and rugged system. The geometry of the optical probe disclosed herein provides a means for making optical measurements in environments where it is difficult and/or expensive to gain access to the vicinity of a measurement point from more than one direction, making it particularly useful for remote sampling operations in industrial environments. Most important, this optical probe geometry allows the measurement location to be moved within the flow stream being monitored while maintaining optical alignment of all optical components thus simplifying alignment and operation of the optical probe.